Some annoying bugs from trying fuel cell models

[valentinsulzer]

• Obscure error if a Variable is included in the variables list but not in the model rhs/algebraic
• Path to parameters doesn't get set right
• process_binary_operators doesn't work with Heaviside operator
• make it obvious that quickplot time is in hours

[valentinsulzer]

• allow different names for spatial variables
• generalize x_average
• ignore discontinuities in the solver if no time dependence